Recently, according to a rapid increase in a high transmission rate for wireless communication, research has been actively conducted to apply multiple input multiple output (MIMO) antenna technology to a transmitter/receiver.
To implement the transmitter/receiver in a MIMO antenna system, an issue of reducing sizes of MIMO antennas and radio frequency (RF) chains and an issue of improving MIMO antenna-based beamforming technology may need to be solved.
To reduce the sizes of the RF chains, a MIMO antenna circuit may need to be provided in a restricted sized, and a performance of a signal processing algorithm may need to be maintained even when degrees of freedom (DoF) of a MIMO antenna decreases in response to an increase in influences of electromagnetic coupling and a spatial correlation between antennas due to the reduced sizes. Also, in an economical aspect, it is necessary to reduce costs for an individual RF chain installed for each antenna.
To solve the above issues, there has been provided a MIMO antenna system in a structure including a single active antenna and passive antennas or parasitic antennas surrounding the active antennas. In this structure, a mutual coupling effect may be realized by connecting an RF chain to only the active antenna and applying a current to the passive antennas. Typical MIMO antenna transmission methods may be directly implemented in the structure, and it is referred to as a beam-space MIMO antenna system.
A multiplexing method of a typical MIMO antenna system may be performed by mapping an individual signal to each antenna. In contrast, the beam-space MIMO antenna system may select a beam from preset beams and transmit a modulated signal in a direction of the selected beam.
In terms of improving the beamforming technology, a beamforming performance has increased according to an increase the number of antennas of the transmitter and receiver to achieve a high array gain, and research on technology for increasing a throughput through beamforming has also been actively conducted.
To apply the technology for increasing the throughput through the beamforming, a feedback on accurate channel information may need to be provided from a receiver to a transmitter. For this reason, the throughput may be restricted based on a channel state and an amount of radio resources assigned to the feedback. Communication technology using the beam-space MIMO antenna system may also be affected by the restricted throughput. Thus, there is a desire for technology of stably increasing a throughput even when a feedback on channel information is not provided in the beam-space MIMO antenna system. Also, when the number of antennas increases, the number of designable antennas may be equal to or less than the increased number of antennas. In this instance, a relatively small number of transmission streams may lead to a presence of reserve beam dimension. Thus, there is a desire for technology of increasing an availability of the reserve beam dimension to achieve a maximum gain of the increased number of antennas.